1. Field of the Invention
The invention relates to high-frequency amplifier circuits, and in particular to monolithic integrated-circuit amplifiers having improved output power capabilities at high frequencies.
2. Description of Related Art
Monolithic integrated-circuit amplifiers have the advantages of small size and low cost. When utilized at high frequencies, however, the maximum output power capabilities of such amplifiers decrease to values which are substantially lower than those at lower frequencies. As used in this patent application, the term high frequency means any frequency above the 3 dB rolloff frequency, which is the frequency at which the gain drops more than 3 dB below the nominal DC open-loop gain of the amplifier. The decrease in maximum output power with frequency is typically attributable to a variety of causes.
One typical cause is a limitation on the peak output current imposed by parasitic inductances of the bonding wires and the integrated circuit leadframe used to supply current to the amplifier. Generally, these inductances cannot be decreased in a cost effective manner.
Another cause of the decrease in output power capability is a reduction in the output resistance R.sub.OUT of an amplifier which occurs at high frequencies. Typically, an amplifier is operated at or near its maximum output current I.sub.o to achieve maximum output power. From the general equation for output power (i.e. P.sub.o =1/2I.sub.o.sup.2 R.sub.OUT), it is apparent that any decrease in output resistance will cause a corresponding decrease in output power.